In producing semiconductor elements, cleaning has been carried out with chemicals to remove contaminants adhered on a surface of a semiconductor substrate including silicon wafer. As the chemicals, cleaning solutions containing mainly hydrogen peroxide are often used. For example, a mixed aqueous solution of hydrochloric acid and hydrogen peroxide, a mixed aqueous solution of sulfuric acid and hydrogen peroxide, a mixed aqueous solution of ammonia and hydrogen peroxide and the like are known.
Particularly, a mixed aqueous solution of ammonia indicating basicity in an aqueous solution and hydrogen peroxide is most widely used since it is effective in removing fine particles adhered on a substrate surface. However, in a basic cleaning solution, there has been a problem that metal impurities including Fe and Cu come to be insoluble and adhere on a substrate to exert a bad influence on a characteristics of a semiconductor element.
In order to solve the problem, high purification of ammonia and hydrogen peroxide themselves has been promoting. Presently, a concentration of metal impurities in ammonia and hydrogen peroxide leads to 1 ppb or below. However, on the contrary, there has been a problem that a high purified cleaning solution comes to be sensitive to metal contamination from the outside and metal contamination from a cleaning vessel etc., causes instability for characteristics of a semiconductor element.
On the other hand, as the other methods for preventing adhesion of metal impurities to a semiconductor substrate, addition of a chelating agent has been suggested. For example, in German Patent Published No. 3822350, a method of using ethylenediaminetetraacetic acid (EDTA) as a chelating agent has been suggested. However, in fact, advantageous effects due to addition of EDTA are scarcely observed. Accordingly, a method for treatment of rendering stable advantageous effects has been desired.